In today's automotive market, there exist a variety of propulsion or drive technologies used to power vehicles. The technologies include internal combustion engines (ICEs), electric drive systems utilizing batteries and/or fuel cells as an energy or power source, and hybrid systems utilizing a combination of internal combustion engines and electric drive systems, as well as pure electric systems. The propulsion systems each have specific technological, financial and performance advantages and disadvantages, depending on the state of energy prices, energy infrastructure developments, environmental laws, and government incentives.
The increasing demand to improve fuel economy and reduce emissions in present vehicles has led to the development of advanced hybrid vehicles, as well as pure electric vehicles. With regard to pure electric vehicles, no ICE is required. Electric vehicles are classified as vehicles having only one energy source, typically a battery or a hydrogen fed fuel cell. Hybrid vehicles are classified as vehicles having at least two separate energy sources, typically an internal combustion engine and a battery linked to an electric traction motor. Hybrid vehicles, as compared to standard vehicles driven by an ICE, have improved fuel economy and reduced emissions. During varying driving conditions, hybrid vehicles will alternate between separate power sources, depending on the most efficient manner of operation of each power source. For example, during most operating conditions, a hybrid vehicle equipped with an ICE and an electric motor will shut down the ICE during a stopped or idle condition, allowing the electric motor to propel the vehicle and eventually restart the ICE, improving fuel economy for the hybrid vehicle.
Hybrid vehicles are broadly classified into series or parallel drivetrains, depending upon the configuration of the drivetrains. In a series drivetrain utilizing an ICE and an electric traction motor, only the electric motor drives the wheels of a vehicle. The ICE converts a fuel source to mechanical energy to turn a generator, which converts the mechanical energy to electrical energy to drive the electric motor. In a parallel hybrid drivetrain system, two power sources such as an ICE and an electric traction motor operate in parallel to propel a vehicle. Generally, a hybrid vehicle having a parallel drivetrain combines the power and range advantages of a conventional ICE with the efficiency and electrical regeneration capability of an electric motor to increase fuel economy and lower emissions, as compared with a traditional ICE vehicle. In addition, hybrid vehicles can incorporate both series and parallel paths. Further, hybrids are often described as being either charge depleting or charge sustaining with reference to a battery pack. Charge-depleting hybrids can be charged off the electrical grid; thus, these hybrids share many of the characteristics of purely electric vehicles. In contrast, the batteries in charge-sustaining hybrids receive all of their electrical charging from the ICE.
Battery packs having secondary/rechargeable batteries are an important component of hybrid vehicle systems, as they enable an electric motor/generator (MoGen) to store braking energy in the battery pack during regeneration and charging by the ICE. The MoGen utilizes the stored energy in the battery pack to propel or drive the vehicle when the ICE is not operating. During operation, the ICE will be turned on and off intermittently, according to driving conditions, causing the battery pack to be alternatively charged and discharged by the MoGen.
State of charge (SOC) and state of health (SOH) are two terms of immediate interest with regard to battery systems. The SOC corresponds to the stored charge available to do work relative to that which is available after the battery has been fully charged; this definition is made precise in the model formulation infra. SOC can be viewed as a thermodynamic quantity, enabling one to assess the potential energy of the system. SOH is a term that is becoming more commonly used within the battery community, but which has not to date been clearly defined. Generally, SOH is used to imply that one can deduce how well the battery system is functioning relative to its nominal (rated) and end (failed) states. For our purposes, we assume that we can represent the SOH if we have a method to identify the impedance spectrum for the battery system over the frequency range of interest in an on-line (or adaptive) manner. Hence, knowing the change in the SOH with time may be viewed as enabling one to assess the increase in irreversible losses that is inherent in the aging of batteries. Thus, the thermodynamics (which are invariant over the battery system's life) allow one to assess the potential energy of the system, and irreversible losses can be assessed once the impedance spectrum is clarified.
A set of parameters can be established to characterize the impedance spectrum. The parameters can be regressed by means of a system identification scheme. The need to regress the value of all parameters that critically impact the impedance spectrum motivates the formulation of a generalized method for the system identification problem. It is noted that to merely regress a limited number of parameters and guess, or use other means to determine, the values of other parameters reduces the accuracy and efficiency in reflecting a battery system. Therefore, it is desirous to have an efficient computing means to calculate all the parameters under a single generalized method or process. The result is that the SOC and SOH of a battery can be better known to a vehicle system operator. Furthermore, it is desirous, as well, to derive a generalized method that can be applied to electric and hybrid-electric vehicles. The generalized method can be used for battery systems, which include lead acid, nickel metal hydride, lithium ion, and other promising battery systems.
As can be appreciated, the state of charge (SOC) and the state of health (SOH) of the battery pack in a vehicle system such as a hybrid vehicle system are important in relation to vehicle efficiency, emissions, and availability. In other words, the SOC and SOH of the battery system constitutes a set of critical elements for a vehicle propulsion system. For example, a vehicle operator or an onboard controller needs to know the conditions of the battery pack for regulating the same.
It is known in the art to use a look up table (LUT) to regulate a battery pack having parameters pre-computed based on a standard vehicle, or an experimental vehicle. A standard vehicle is a vehicle other than the vehicle which a vehicle operator is handling. A difficulty with the prior art approaches is that they are either not vehicle specific, or lack a generalized approach to multiple parameter handling, thereby reducing the utility of the prior art systems. In addition, it is known in the art to use Coulomb counting to obtain a SOC value of a battery system. Coulomb counting is easily implemented provided the current efficiency is known precisely for all times and conditions. Because this is not normally the case, the voltage signal can be used in a model that incorporates the voltage for determining the SOC.